The field of the present disclosure relates generally to Rankine cycle devices. More particularly, the present disclosure relates to systems and methods for cold startup of Rankine cycle devices.
Rankine cycle devices use a working fluid in a closed-loop cycle to gather heat from a heating source, or a heat reservoir, by generating a hot gaseous stream. The hot gaseous stream is expanded through a turbine to generate power, typically electrical power. The expanded stream is then condensed in a condenser by transferring heat from the expanded stream to a cold reservoir. The working fluid remains in a closed loop and is repeatedly sent through the Rankine cycle.
The efficiency of Rankine cycle devices, such as Organic Rankine Cycle (ORC) devices, in a low-temperature heat recovery application is sensitive to temperatures of the hot and cold reservoirs between which they operate. In an ORC device, the working fluid is an organic, high molecular mass fluid with a liquid-vapor phase change, or boiling point, occurring at a lower temperature than the water-steam phase change point. Typically, the temperatures of the reservoirs change significantly during the lifetime of the plant. Geothermal plants, for example, may be designed for a particular temperature of a geothermal heating fluid from the earth, but lose efficiency as the ground fluid cools over time. Air-cooled ORC plants that use an exhaust at a constant temperature from a larger plant as heating fluid deviate from their designed operating conditions as outside air temperature changes.
Typically, ORC plant designs encounter unreliable cold startup conditions because the working fluid condenses and settles inside the loop, rather than in the feed vessel, after the ORC plant shuts down. Plant start-up thus may become difficult, or fail altogether, with the working fluid blocking the expansion device during cold startup conditions.